It's been a good week for the Santa Marta harlequin frog. Like some forgotten Radio 1 DJ, long-departed soap star or C-list celebrity, it has reappeared on the front pages after an absence of 14 years. But this is no publicity stunt. For along with its cousin, the San Lorenzo harlequin frog, this South American amphibian has been rediscovered, after having been feared extinct.

The discovery was made in a reserve in the remote Sierra Nevada de Santa Marta, on the Caribbean coast of Colombia. In some ways it is hardly surprising that the frogs should have evaded detection for so long. Finding an individual species - especially such a rare one - in such a far-flung location is the biological equivalent of seeking needles in haystacks.

The frogs are the not the first creatures to have been rediscovered decades, centuries, or even millions of years after they were considered extinct. But their reappearance does raise questions about what we mean when we categorise a species as "extinct". Negatives are notoriously hard to prove, especially for an elusive animal in a remote, inaccessible environment - which explains why most of these "back from the dead" examples are small creatures such as rodents, amphibians or small birds living in dense jungles. We do occasionally know the exact moment when a species shuffled off this mortal coil: the last surviving Stephen's Island wren was brought in by the lighthouse-keeper's cat, Tibbles, in 1894. But otherwise we have to rely on a mixture of guesswork, methodical surveys and luck.

The paradox is that even as we rediscover supposedly "lost" creatures, and discover previously unrecorded ones, the rate of extinction is increasing faster than at any time in recorded history. Back in Colombia, our newly found frogs run the risks of pollution, deforestation and, of course, climate change. And if that's not enough, a skin fungus has devastated frog populations just 40km from their home. Perhaps they should make the most of their day in the limelight - it may be their last.

· Stephen Moss is a producer at the BBC Natural History Unit and author of Everything You Always Wanted to Know About Birds - But Were Afraid To Ask! (Christopher Helm, £9.99).

Additional reporting by Mel Bezalel

Tarpan horse Poland

Not all creatures that come back from the dead do so under their own steam. Recent advances in genetic science have brought closer the possibility of recreating extinct animals from remnants of DNA. But another, less complicated technique is to "back-cross" descendants of the original animal. This method works particularly well with domesticated animals, where even though the pure-bred strain may have disappeared, enough crossbreeds and hybrid descendants exist to make the process viable. Thus, as long ago as 1933 German scientists "recreated" the Tarpan (or Przewalksi's) horse, a prehistoric European species that had died out in the late 19th century. Today, Tarpans are thriving in the Biebrza marshes of central Poland, where their regular grazing helps to preserve a unique wetland habitat.

Coelacanth Kenya

This prehistoric fish is the big daddy of all rediscovered creatures, whose story sums up the unpredictability of nature. The coelacanth had been extinct for 65m years when an alert museum curator in South Africa noticed a "strange, blue fish" in a catch she was inspecting in 1938. News of the discovery travelled around the world. Since then, two more populations of coelacanths have been found, off Madagascar and Indonesia, though the species is still considered highly endangered. Recently, however, divers off the coast of South Africa found six coelacanths - the first sighting in the region since the original finding.

Ivory-billed woodpecker United States

"It's kind of like finding Elvis," was the reaction of one excited American ornithologist. He was referring to the apparent rediscovery of the continent's largest woodpecker, in the swamps of Arkansas, after an absence of 60 years. But the celebrations may have been premature: David Sibley, author of the definitive Sibley Guide to the Birds of North America, has called the video evidence of the bird into question and believes the finders saw the smaller, much more common pileated woodpecker. Now the only hope of finding an ivory-bill alive rests on expeditions to the mountain forests of eastern Cuba, where it was last sighted in the 1980s.

Bavarian pine vole Aka Bavarian short-eared mouse

In 1962, a German zoologist encountered a rare species of rodent in the Bavarian Alps; in August 2003, another scientist exploring an isolated spruce forest found it again. During the intervening 40 or so years, there were no reliable sightings. In some ways, this is hardly surprising: the Bavarian short-eared mouse (which despite its name is actually a vole) closely resembles many other small rodents - and as the scientist who found it says, "All voles look like sausages with four legs." So long as its habitat remains undisturbed, its prospects look pretty hopeful.

Lord Howe Island stick insect Australia

Named after an 18th-century first lord of the admiralty, Lord Howe Island is located to the east of Australia, roughly equidistant between Brisbane and Sydney. Before European settlers arrived in 1834, the island was home to a unique set of native species. But, as with so many oceanic islands, once humans and their attendant cargo of rats, cats and other livestock arrived, these creatures were doomed. One creature thought lost to rats was the Lord Howe Island stick insect, but it was rediscovered in 2001 after an absence of more than 80 years. A tiny population of these nocturnal creatures was found surviving on Balls Pyramid, a rocky outcrop 23km offshore. As one of the finders said, "We couldn't jump for joy for fear of falling off!"

Snowdonia hawkweed Wales

Being nibbled to death by sheep is an occupational hazard of mountain plants; but for one Welsh speciality it almost resulted in disaster. In the early 1950s, the Snowdonia hawkweed, a perennial with bright yellow flowers, was believed to have become extinct as a result of overgrazing. But in 2002, almost 50 years after it had last been seen, botanists discovered the plant on steep slopes near Bethesda - ironically, exactly where it had last been seen, and where it had been searched for without success. To try to prevent the plant vanishing once again, conservationists have removed sheep from the immediate area, and have collected seed as an insurance policy just in case.